1. Field of the Invention
The present invention relates to solid-state image-pickup devices, methods of driving a solid-state image-pickup device, and image-pickup apparatuses.
2. Description of the Related Art
For an image-pickup apparatus, a such as digital still camera, a video camera, or a camera module installed in a portable information terminal, e.g., mobile phone, a solid-state image-pickup device, such as a charge coupled device (CCD) image sensors or a complementary metal oxide semiconductor (CMOS) image sensor, is used as an image input device (image-pickup device).
In general, an image-pickup apparatus has an autofocus (AF) function in which the apparatus is automatically focused on an object. There are various known autofocus methods, such as a phase difference method in which focusing is carried out by measuring the distance to an object with two sensors using the phase difference of the image of the object and a contrast method, also known as a mountain-climbing method, in which a point with the greatest contrast is brought to focus.
An image-pickup apparatus that does not have a special sensor for autofocus, such as an image-pickup apparatus having an autofocus function according to the contrast method, carries out various types of processing, such as generating a captured image for recording on the basis of information from the solid-state image-pickup device, generating a monitoring image to be displayed on a monitor, such as a liquid crystal display (LCD) or a electronic view finder (EVF), and detecting contrast for autofocus.
To generate a monitoring image or to detect contrast for autofocus according to the contrast method, processing can be carried out by using only part, and not all, of the pixel information from the image-pickup device. Therefore, in general, part of the pixel information obtained by thinning the pixel information from the solid-state image-pickup device is used to reduce the processing load and electric consumption.
In the autofocus process according to the contrast method, data sets corresponding to several fields are captured, and a peak value is detected by comparing the data sets. For this reason, more time is required for the processing compared with the time required for autofocus according to the phase difference method. More specifically, since monitoring image generation process and the contrast detection process are carried out simultaneously on the basis of pixel information obtained at one solid-state image-pickup device, the autofocus processing cycle is limited to the cycle V of the monitoring image (i.e., field cycle of 50 Hz or 60 Hz).
As a known method to speed up the autofocus process, a fast AF mode in which the cycle of one field is shortened by reading out information from only part of the image-pickup device after the shutter button is pressed or before static image exposure in continuous shooting is provided (for example, refer to Japanese Unexamined Patent Application Publication No. 2001-296470).
Furthermore, as image signal readout modes, in addition to a draft mode in which image signals corresponding to a predetermined number of pixels among all pixels of the image-pickup device, an automatic focusing mode in which a plurality of images is read out at high speed by reading out image signals corresponding to a number of pixels less than that read out in the draft mode is provided (for example, refer to Japanese Unexamined Patent Application Publication No. 2004-023747).